1. Field of the Invention
The present invention relates to a field emission-type electron source for emitting electron beams by means of the field emission phenomenon.
2. Description of the Prior Art
Heretofore, there has been well known in the art a field emission-type electron source (hereinafter referred to as xe2x80x9celectron sourcexe2x80x9d for brevity), as disclosed, for example, in Japanese Patent Publication No. 2987140 and Japanese Patent Laid-Open Publication No. 2001-126610.
The electron source disclosed in Japanese Patent Publication No. 2987140 includes a lower electrode, a surface electrode (upper electrode) composed of a metal thin-film, and an electron transit layer (strong-field drift layer) provided between the lower and surface electrodes. The surface electrode is disposed opposed to the lower electrode with interposing the electron transit layer therebetween. When a certain voltage is applied between the lower and surface electrodes such that the surface electrode has a higher potential than that of the lower electrode, the resulting electric field between the electrodes induces the flow of electrons from the lower electrode to the surface electrode through the electron transit layer.
For example, an oxidized or nitrided porous polycrystalline silicon layer is used as the electron transit layer. In activating the electron source, a collector is positioned opposed to the surface electrode while exposing the surface electrode to a vacuum space. Then, a DC current is applied between the surface and lower electrodes such that the surface electrode has a higher potential than that of the lower electrode, and simultaneously another DC current is applied between the collector and surface electrodes such that the collector has a higher potential than that of the surface electrode. As a result of the above operation, electrons are injected from the lower electrode to the electron transit layer, and then emitted through the surface electrode after drifting within the electron transit layer. Alteration such as oxidation of the surface of the surface electrode causes deterioration in the efficiency of emitting electrons or the electron emitting efficiency. Thus, the surface electrode is typically made of chemically stable metal (e.g. noble metal such as gold). In this case, the thickness of the surface electrode is set at about 10 nm.
In electron sources, the terms xe2x80x9cdiode current Ipsxe2x80x9d and xe2x80x9cemission current (emission electron current) Iexe2x80x9d generally mean a current flowing between the surface and lower electrodes and a current flowing between the collector and surface electrodes, respectively. Greater ratio of the emission current Ie to the diode current (Ie/Ips) provides higher electrode emitting efficiency [(Ie/Ips)xc3x97100(%)]. The above electron source can emit electrons even if the DC voltage to be applied between the surface and lower electrodes is in a low range of about 10 to 20 V. Thus, the electron source can exhibit enhanced electron emission characteristics having a low dependence on vacuum degree, and emit electrons stably without occurrence of the so-called popping phenomenon during electron emission.
The lower electrode of the electron source comprises a semiconductor substrate having a resistivity relatively close to that of a conductor, and an ohmic electrode formed on the back surface of the semiconductor substrate. Alternatively, the lower electrode comprises an insulative substrate and a metal conductive layer formed on the insulative substrate.
On the other hand, the electron source disclosed in Japanese Patent Laid-Open Publication No. 2001-126610 comprises a surface electrode partially including a carbon region made of carbon or carbon compounds. This electron source can advantageously prevent excessive diode current Ips to provide enhanced electron emitting efficiency.
The conventional electron sources as described above are used in a vacuum-sealed state. In this connection, an assembling process of such electron sources involves a relatively high-temperature thermal process including a vacuum-sealing process (which is performed at about 500xc2x0 C.), which inevitably increases the electrical resistance of the surface electrode and/or the lower electrode. The increased electrical resistance makes it difficult for voltage to be adequately applied between the surface and lower electrodes and/or between the collector and surface electrodes, resulting in deteriorated electron emission characteristics (such as lowered emission current or electron emitting efficiency).
The surface electrode is composed of a metal thin-film. The thickness of the surface electrode is set at 10 nm as in the foregoing electron source. A gold thin-film commonly used as the surface electrode is involved with agglomeration at a temperature of 400xc2x0 C. or more. The resulting agglomeration deteriorates the thickness uniformity and continuity of the thin film, and thereby the electrical resistance of the surface electrode is increased, resulting in deteriorated electron emission characteristics. Tungsten and aluminum are known as metal having high resistibility to such agglomeration. However, if the surface electrode is made of tungsten or aluminum, the surface of the electrode will be subject to oxidation, which leads to deteriorated electron emitting efficiency.
Similarly, the lower electrode has an increased electrical resistance. The following factors are assumed as the reason for the increased electrical resistance.
(1) Agglomeration of metal
(2) Decrease in the film thickness of the lower electrode due to thermal diffusion of atoms (metal atoms) constituting the lower electrode toward a layer (e.g. electron transit layer) deposited on the lower electrode
(3) Decrease in the film thickness of the lower electrode due to thermal diffusion of atoms constituting a layer (e.g. electron transit layer) deposited on the lower electrode toward the lower electrode
(4) Increase in resistivity of the lower electrode
In addition to the aforementioned electron sources, various modified electron sources for emitting electrons by means of the field emission phenomenon have been proposed. For example, one of the proposed electron sources has a MIM (Metal-Insulator-Metal) structure including an insulating layer serving as the electron transit layer. Another electron source has a MIS (Metal-Insular-Semiconductor) including a semiconductor layer provided between the electron transit layer and the lower electrode.
In view of industrial availability of the various electron sources, it is desired to provide increased emission current and enhanced electron emitting efficiency contributing to reduction in power consumption. In all of the above electron sources, electrons are emitted through the surface electrode. Thus, the electron emitting efficiency can be increased by reducing an energy loss due to electron scattering within the surface electrode. From this point of view, it is contemplable to reduce the thickness of the surface electrode insofar as device characteristics are not adversely affected. For example, Japanese Patent Laid-Open Publication No. 2001-243901 discloses an electron source intended to provide enhanced electron emitting efficiency based on the above approach. In this electron source, a surface electrode includes a metal thin-film portion having a flat surface, and a plurality of island-shaped raised portions protruding from the surface of the metal thin-film portion, the raised portions being continuously and integrally formed with the thin-film portion. However, the respective metal raised portions of this electron sources are connected with each other by the metal thin-film portion, and thereby the film thickness of the metal thin-film portion will impose a restriction on a lower limit of the electrical resistance of the surface electrode. Thus, even if it is tried to achieve a specific electrical resistance of the surface electrode required for device characteristics, there exists such an disadvantage that the thickness of the metal thin-film portion cannot be sufficiently reduced or the electron emitting efficiency cannot be sufficiently enhanced.
In view of the problems of the conventional electron sources, it is therefore an object of the present invention to provide an electron source capable of suppressing deterioration of electron emission characteristics and achieving enhanced thermal resistance.
It is another object of the present invention to provide an electron source capable of suppressing the increase of electrical resistance in a surface electrode and achieving enhanced electron emission characteristics.
In order to achieve the above objects, according to one aspect of the present invention, there is provided an electron source (field emission-type electron source) including a lower electrode, an electron transit layer, and a surface electrode. The electron transit layer is formed on the lower electrode and composed of a composite nanocrystal layer including polycrystalline silicon and a number of nanocrystalline silicons residing adjacent to a grain boundary of the polycrystalline silicon. The surface electrode is formed on the electron transit layer. In this electron source, an electron passing through the electron transit layer is emitted through the surface electrode. At least a portion of the surface electrode and/or at least a portion of the lower electrode are made of layer-structured conductive carbide or layer-structured conductive nitride.
The conductive carbide or conductive nitride has a relatively high conductivity, a relatively low work function, a melting point higher than noble metal, and an excellent diffusion barrier performance. In addition, the conductive carbide or conductive nitride has an oxidation resistance superior to that of metal such as tungsten or aluminum. In the electron source of the present invention, at least a portion of the surface electrode and/or at least a portion of the lower electrode are made of layer-structured conductive carbide or layer-structured conductive nitride. Thus, as compared to an electron source having a surface electrode composed of a metal thin-film, the electron source of the present invention can provide enhanced thermal resistance of the surface electrode and/or the lower electrode while suppressing the deterioration of electron emission characteristics. This makes it possible to prevent electron emission characteristics from being deteriorated due to a thermal process such as a vacuum-sealing process.
According to another aspect of the present invention, there is provided an electron source comprising a lower electrode, a surface electrode, and an electron transit layer. The electron transit layer is interposed between the lower electrode and the surface electrode to allow an electron to pass therethrough. An electron passed through the electron transit layer is emitted through the surface electrode. The surface electrode is composed of a multilayer film including a conductive carbide or conductive nitride layer and a noble metal layer, the surface electrode having a surface formed with a number of concave portions allowing the multilayer film to have a locally reduced thickness.
In this electron source, an energy loss due to electron scattering will be reduced in the region of the surface electrode formed with the concave portions as comparted to other region. Further, electron emission characteristics can be enhanced while suppressing the increase of electrical resistance in the surface electrode because the electrical resistance of the surface electrode is substantially determined by the film thickness of the multilayer film composed of a conductive carbide layer or conductive nitride layer and a noble metal layer.